A guide bearing of a rotating shaft usually comprises two internal and external rings tightening rolling units, the external ring being mounted on a fixed portion and the internal ring being mounted on a cylindrical contact surface of the shaft. The internal ring of the guide bearing can be locked axially at one end on an annular radial edge of the cylindrical contact surface of the shaft and at the other end by a screwed element on a threaded portion of this cylindrical contact surface.
In known embodiments, the element that provides by one end the tightening of the internal ring on the edge of the cylindrical contact surface, comprises means for sealing the enclosure containing the guide bearing and the oil in suspension. For this, this element comprises for example labyrinth seals interacting with tracks of abradable material borne by a wall of this enclosure.
When the shaft guided in such a bearing is a shaft of a high-pressure compressor of a turbojet, it is connected by a pair of bevel gears (IGB or Inlet GearBox) to a radial drive shaft of certain equipment comprising in particular a fuel pump. The guide bearing of the shaft of the compressor and the pair of bevel gears (IGB) form a modular unit which is provided assembled by its supplier before its assembly in the turbojet.
This module comprises an upstream wedge placed between the annular edge of the cylindrical contact surface of the shaft and the internal ring of the bearing, which must be in contact on this edge. This wedge is re-machined before the assembly of the module, in such a way as to correctly position the bevel gear of the shaft of the compressor in relation to the bevel gear of the radial shaft, in order to provide proper gear mesh of the pair of bevel gears.
This module must provide, without re-machining during the engine assembly, correct positioning of the rotor of the high-pressure compressor in relation to its stator.
For this purpose, the module provided assembled must comply with a precise tolerance, an axial dimension between the laying plane of the support of the pair of bevel gears (IGB) and a radial face of the screwed element which forms an axial abutment of the shaft of the high-pressure compressor. The tolerance on this axial dimension is much less than the sum of the tolerances of the corresponding parts (the support of the pair of bevel gears, the guide bearing of the shaft and the screwed element), in such a way that the supplier of the module must re-machine one of these parts in order to comply with the tolerance on the axial dimension of the unit.
It is in general the upstream end of the screwed element that is applied on the internal ring of the bearing, that is re-machined. Another possibility is to place a wedge between the internal ring of the bearing and the screwed element. This leads to the supplier of the module unscrewing and rescrewing this element in the module, which is a delicate operation due to the low threaded length of this element, of the low radial dimension of the threads and of the risk of damaging the wipers of the labyrinth seals of this element by the tools used.
Indeed, the use of a tool for screwing the element can damage the wipers of the labyrinth seals, leading to a decrease in the effectiveness of the sealing of the labyrinth seals, which causes a lubricating oil leak of the enclosure containing the guide bearing.
Furthermore, the low dimensions of the threadings of the cylindrical contact surface and of the screwed element result in a poor centering of the element on the cylindrical contact surface. As such, the clearance required between the labyrinth seals and the abradable tracks required to pressurise the enclosure containing the guide bearing, cannot be obtained, which results in a premature wear of the abradable tracks as well as in a poor sealing of the enclosure of the guide bearing.